A protocol has been designed to provide answers to the following questions: (1) does moderate restriction of the intake of protein and phosphorus slow progression of early chronic renal failure (serum creatinine [Cr] 2-5 mg/dl)? (2) does more severe restriction of the intake of protein and phosphorus plus supplementation with either essential amino acids or a ketoacid/amino acid mixture slow the progression of renal failure at a later stage (Cr 5-7 mg/dl)? A multicenter trial involving up to 15 centers is proposed; detailed budgets are submitted for three representative clinical centers and for a core laboratory. Rates of progression will be determined by repetitive measurements of isotopically determined glomerular filtration rate. In Stage I, non-nephrotic patients with Cr = 2-5 mg/dl will be randomized to either no treatment or a diet containing 0.55 gm/kg protein of predominantly high biological quality and 600-800 mg phosphorus. Stage I continues until Cr is greater than 5 mg/dl, at which point they enter Stage II. Patients first identified when Cr is 5-7 mg/dl will enter Stage II directly. In Stage II, patients are randomized to receive either (1) the same regimen as Stage I; (2) a diet containing 0.28 gm/kg of mixed quality protein and approximately 500 mg phosphorus, plus 1-1.5 gm supplemental calcium plus vitamin supplement plus 15 mg/day of essential amino acids; or (3) the same except that amino acids are replaced with a ketoacid/amino acid mixture, 18 gm/day. All patients will be carefully followed to assess any signs of protein or phosphate deficiency, by bimonthly determinations of serum proteins (including albumin and transferrin) and electrolytes, and determination of 24-hr urinary excretion of urea, creatinine, phosphorus, and protein. 25-OH- and 1,25-OH-vitamin D and N-terminal parathyroid hormone will be measured quarterly. compliance with the protein prescription will be assessed from urea excretion, and compliance with ketoacids by monitoring plasma alloisoleucine. Statistical methods will be employed to determine whether rates of progression assessed from serial determinations of GFR and creatinine differ between the two groups of Stage I and between the three groups of Stage II, and also whether progression in individual patients during Stage II differs from rates of progression assessed from the same measurements in Stage I. Results will be related to the extent of reduction in protein intake in each subject in the transition from Stage I to Stage II (as revealed by urinary urea measurements).